Nanoindentation responses near single grain boundaries in oxide ceramics

Local mechanical responses near single grain boundaries (GBs) of 9.8 mol% Y 2 O 3 ‐stabilized ZrO 2 bicrystals, SrTiO 3 bicrystal, and Al 2 O 3 polycrystal were investigated by nanoindentation at room temperature to reveal single GB contributions to the mechanical properties. Contrary to the concept of the Hall–Petch strengthening, the hardness showed negligible variations among grain interiors, GB vicinities, and just on GBs in the samples. Importantly, the hardness may be underestimated owing to GB grooving in thermally etched samples. Despite the negligible change in intrinsic hardness, the transmission electron microscopy observation of the ZrO 2 specimen under impressions revealed dislocation pileups at GBs. These results suggest that single GBs of oxide ceramics prevented dislocation movement but with limited strengthening contributions. This should correspond to the macroscopic Hall–Petch behaviors in oxide ceramics, where the Hall–Petch coefficients normalized by friction stress are significantly smaller than those in metals.